I. Field of the Invention
The present invention relates generally to control of frequency and timing, and more particularly to a method for determining and compensating for frequency errors in reference oscillators used in receivers in communication systems. The invention further relates to a technique for determining and compensating for Doppler effects caused by relative motion between transmitters and receivers.
II. Description of the Related Art
Typical advanced terrestrial communication systems, such as wireless data or telephone systems, use base stations, also referred to as cell sites, within predefined geographical regions or cells, to relay communication signals to and from one or more user terminals or system subscribers. Satellite-based communication systems use base stations referred to as gateways, and one or more satellites to relay communication signals between the gateways and one or more user terminals. Base stations and gateways provide communication links from each user terminal to other user terminals or users of other connected communication systems, such as a public telephone switching network. User terminals in such systems can be fixed or mobile, such as a mobile telephone, and positioned near a gateway or remotely located.
Some communication systems employ code division multiple access (CDMA) spread-spectrum signals, as disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", and U.S. patent application Ser. No. 08/368,570, filed Jan. 4, 1995, entitled "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy," which are both assigned to the assignee of the present invention, and are incorporated herein by reference.
In a typical spread-spectrum communication system, one or more preselected pseudonoise (PN) code sequences are used to modulate or "spread" information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN code spreading, a method of spread-spectrum transmission that is well known in the art, produces signals for transmission with a bandwidth much greater than that of the data signal. In a base station- or gateway-to-user terminal communication path or link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or over different beams, as well as between multipath signals. This is also referred to as the forward link.
In a typical CDMA spread-spectrum system, channelizing codes are used to differentiate signals intended for various user terminals within a cell or a satellite sub-beam on the forward link. Each user transceiver has its own orthogonal channel provided on the forward link by using a unique "channelizing" orthogonal code. Signals transferred on these channels are generally referred to as "traffic signals." Additional forward link channels or signals are provided for "paging", "synchronization", and other signals transmitted to system users. Walsh functions are generally used to implement the channelizing codes.
Additional details of the operation of this type of transmission apparatus are found in U.S. Pat. No. 5,103,459, entitled "System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone," assigned to the same assignee as the present invention and incorporated herein by reference.
CDMA spread-spectrum communication systems, such as disclosed in the above patents, contemplate the use of coherent modulation and demodulation for forward link user terminal communications. In communication systems using this approach, a "pilot" carrier signal, or simply a "pilot signal," is used as a coherent phase reference for forward link signals. A pilot signal is a signal which generally contains no data modulation, and is transmitted by a gateway, or base station, throughout a region of coverage as a reference.
Pilot signals are used by user terminals to obtain initial system synchronization and time, frequency, and phase tracking of other signals transmitted by base stations or gateways. Phase information obtained from tracking a pilot signal carrier is used as a carrier phase reference for coherent demodulation of other system signals or traffic (data) signals. This technique allows many traffic signals to share a common pilot signal as a phase reference, providing for a less costly and more efficient tracking mechanism. A single pilot signal is typically transmitted by each base station or gateway for each frequency used, referred to as CDMA channels or sub-beams, and shared by all user terminals receiving signals from that source on that frequency.
When user terminals are not receiving or transmitting traffic signals, information can be conveyed to them using one or more signals known as paging signals or channels. For example, when a call has been placed to a particular mobile phone, a base station or gateway alerts that mobile phone by means of a paging signal. Paging signals are used to designate the presence of a call, which traffic channel to use, and to also distribute system overhead information, along with system subscriber specific messages. A communication system may have several paging signals or channels. Synchronization signals can also be used to transfer system information useful to facilitate time synchronization. All of these signals act as shared resources in a manner similar to pilot signals.
User terminals can respond to a message on a paging signal by sending an access signal over the reverse link. That is, the signal path from the user terminal to the base station or gateway. Access signals are also used by user terminals when they originate calls, and are sometimes referred to as access probes. In addition, additional long PN codes, which are not orthogonal, are typically used to create reverse link traffic channels. At the same time, a form of M-ary modulation using a set of orthogonal codes can be used to improve reverse link data transfer.
As with any communication system, forward link communication signals are received by the user terminal and downconverted into a baseband frequency for further processing. Once downconverted, the signals are processed digitally to detect the particular pilot signal or signals being received, and to demodulate associated paging, synchronization, and traffic signals. For spread spectrum systems, the PN spreading codes are applied during demodulation to despread the signals and channelizing codes are correlated with the signals to render data.
In order for the reception, downconversion, and demodulation processing to work correctly in such systems, the user terminal must share a common frequency reference with base stations or gateways transmitting the signals being processed. That is, because information is carried in the phase of the signal carrier, the carrier frequency must be accurately detected, and the position of relative phases of multiple carriers must also be determined. Without reasonably accurate frequency tuning, the carrier cannot be properly removed and the digital signals accurately despread and demodulated.
PN spreading codes and orthogonal channelizing codes cannot be accurately removed without appropriate system timing or signal synchronization. If the codes are applied with incorrect synchronization, the signals will simply appear as noise and no information is conveyed. Determining the positions of satellites, user terminals, and code timing offsets used in such systems, also depends on an accurate knowledge of time or relative temporal displacement. User terminals rely on the accuracy of local oscillators to maintain an appropriate clock rate, event timing, and relative time values with respect to base station or gateway timing, and absolute chronological history or relationships.
To aid this process, local oscillator frequency sources in user terminals can be made to operate with high precision, or can incorporate highly advanced timing circuits or frequency generators. Receivers can be added to detect "universal time" for maintaining chronological accuracy, such as through the use of known GPS system signals. However, such elements are generally undesirable for several reasons. Firstly, their material or manufacturing cost is prohibitive for use in many commercial applications such as for cellular telephones. Secondly, their complexity affects user terminal reliability, especially for typical commercial environments. In addition, power consumption may be increased with more complex or specialized circuits, which negatively impacts power cell life for portable communication devices.
The output frequency of reference sources could also be checked and adjusted or tuned using various forms of feedback control. However, communication systems employing satellites with non-geostationary orbits, exhibit a high degree of relative user terminal and satellite motion. This creates fairly substantial Doppler shifting in the apparent carrier frequency of signals within the communication links. Such Doppler effects must also be accounted for when determining oscillator error, or drift during use, and reduces the usefulness of conventional phase locked loops and other feedback controls. Again, undesirable complexity is needed to implement solutions. The same is also true for non-satellite based communication systems communicating with mobile user terminals or other types of moving repeater platforms that move at high speeds.
Therefore, any system desiring to detect drift or inaccuracies in oscillator output frequencies must also be able to account for Doppler effects on signals being transferred. Unfortunately, while the relative motion between gateways and satellites are well defined, the motion between satellites and user terminals is not. Current communication system designs have been unable to account for the impact of Doppler due to this latter motion, especially in the presence of contemporaneous oscillator errors.
One technique used to help compensate for Doppler or oscillator errors, is to employ what are referred to as deskew buffers which temporarily store a portion of received signals so they can be shifted in time. The size and storage capacity of deskew buffers defines limits on the amount of frequency offset or error for which they can compensate. Buffer sizes are limited by well known cost and circuit design factors. Unfortunately, for large amounts of Doppler shifting, the amount of signal storage needed to compensate exceeds the typical deskew buffer capacity. In addition, if a user terminal oscillator drifts sufficiently, or continues to drift during communication, which is likely for systems using inexpensive oscillators, frequency errors also exceed the deskew buffer capacity and communication link synchronization is lost.
Therefore, what is needed is a method and apparatus for separating and determining both oscillator accuracy or frequency tuning errors for, and Doppler effects experienced by, user terminals within a communication system. This should be accomplished very reliably without undue complexity or cost. It is especially desirable to measure and account for Doppler effects occurring between user terminals and satellites relaying communication signals.